There are 2 things taught to us:variables(litersls ) and constants . If we let a value 'a' as a constant ,meaning it would remain unchanged and also it is a set of real numbers . If we take 'x' as a variable, meaning it can change and it is also a set of real nos. For ex. Lim (x>a) x = a ,a:fixed value ,any value from end to end, but still it is not getting clear to me about 'x'?? If it does change then w.r.t to what ?